King Arthur is the enigmatic ruler of the Western Planet and serves as the primary antagonist in the narrative. His character is defined by mystery, deception, and a tyrannical ambition that starkly contrasts with the legendary heroic figure from traditional folklore.

Very little is known about Arthur's true background, which is a deliberate aspect of his character. He presents himself as a charismatic and almost divine leader who has successfully united the entirety of the Western Planet under his rule. His ultimate goal is to conquer the Eastern Planet, driven by a desire to obtain the Holy Grail. Arthur is the figure whom most of the Western forces believe to be the true Savior King, a prophecy that drives much of the conflict in the story.

In terms of personality, Arthur is deceptive, calculating, and manipulative. While he projects an image of a wise and benevolent savior, clad in gold and white robes that suggest divinity, this facade hides a far more sinister nature. He is depicted as a tyrant who gathers followers who are often as morally questionable as he is. His authority is maintained not through genuine loyalty, but through supernatural manipulation, revealing a core of callous ambition beneath his serene exterior.

Arthur's role in the story is that of a powerful and seemingly untouchable sovereign from his flying throne on the Western Planet. He does not typically engage in direct combat himself, instead commanding his loyal and formidable Round Table of generals to do his bidding. He orchestrates the invasion of the East and is the central obstacle that the protagonist, Oda Nobunaga, must overcome to bring peace to both worlds. His presence as the purported Savior King creates a direct ideological and physical conflict with Nobunaga, who is prophesied by others to be that same savior.

A key aspect of Arthur's character is his profound manipulation of key figures, which defines his most important relationships. He maintains a round table of powerful warlords from different eras, such as Julius Caesar and Alexander the Great, who are fanatically loyal to him. This loyalty is not earned through trust or shared ideals but is the result of Arthur's unique supernatural ability. This power allows him to present a different face to each person he meets, showing them exactly what they most desire to see. To Caesar, he appears as a beautiful woman, while to Alexander, he takes the form of a man who resembles the mythical hero Achilles. His ability to exploit the deepest desires of powerful individuals ensures their unwavering servitude. Later in the story, he also successfully manipulates Akechi Mitsuhide, a close ally of Nobunaga, by appearing to him as his deceased father, thus turning him against the protagonist.

While Arthur does not undergo significant personal development or redemption, his true nature is gradually revealed as the story progresses. The mystery surrounding him deepens until his final confrontation, where his true form is exposed not as a radiant king, but as a decrepit old man. This revelation shatters the illusion he has carefully maintained, marking his transition from a seemingly divine being to a vulnerable tyrant. His end comes during a climactic battle between Nobunaga and Alexander, where he is crushed by the avatars of the Star Dragons while attempting to summon the Holy Grail.

Arthur's most notable ability is his power of appearance alteration, a form of perception manipulation that makes him functionally faceless, as his true features are obscured by an eerie light. This power is fundamental to his role as a villain, as he uses it not for combat but for psychological domination, turning potential enemies into devoted followers. Beyond this, he possesses the resources and authority of a king who has united an entire planet, and his quest for the Holy Grail demonstrates a desire for ultimate, reality-altering power.